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evolution

This lone tree may be in a fixed position all its life… but its babies will probably grow somewhere else!

Seed dispersal

There is no animal alive that eats the avocado seed, and its too big to be transported by wind. It is believed to have evolved to be carried by a now extinct species.

Dispersal is the process of organisms spreading from one place to another. All species, including plants, maintain or expand their range through dispersal. Dispersal is necessary because new generations cannot easily occupy the same physical space their parents occupy.

Unlike most animals, plants have limited abilities to seek out favorable environments. Consequently, plants have evolved adaptations for dispersing by means of seeds, spores or vegetative outgrowths.

Plants overcome their little capacity to find favorable environments by producing lots of seeds, ensuring enough of them will settle at favorable sites. Weeds have small seeds, which are better for long-distance travel. Small seeds, however, contain only a limited food supply for the germinating seedling. So there are trade offs between dispersal distance and the food supply. Larger seeds generally have shorter dispersal distances because of their weight.

Plants have evolved dispersal mechanisms that take advantage of various forms of kinetic energy, including gravity, wind, the flow of water and the movement of animals. There is also ballistic/mechanical dispersal, where a seed pod explodes open and flings its seeds away from the mother plant. Read on and you’ll learn all about it!

Dispersal by water

Aquatic and wetland species sometimes use water to disperse their fruits. The coconut for example, produces a large, dry, fiber-filled fruit capable of surviving adrift at least for long periods because of its air pockets.

Mangrove seeds germinate while attached to the adult plant and seedlings float away on the surface of the water.

The largest seed in the plant kingdom comes from the coco-de-mer palm, native to the Seychelles Archipelago in the Indian Ocean about 1,000 miles East of Kenya.

The coco-de-mer seed weighs up to 18kg but can float and is dispersed by water.

Dispersal by wind

Various wing or propeller-like structures have evolved to slow the descent of fruits after they attach from their parent. Other plants produce lightweight seeds that are adapted to rise upwards in updrafts of air.

The dandelion uses “lighter-than-air” dispersal. Feathery bristles function like a parachute in the wind.

Maple fruit have flattened wings of fibrous, papery tissue that allow them to flutter in the wind.

Here’s an awesome example of a “Cattail” plant that uses wind dispersal.

Dispersal by animals

There are two types of seed dispersal by animals:

One is when the fruit has a multi-layered fleshy and flavorful covering. This is meant to entice animals to eat the fruit. If eaten by certain animals, the seeds inside the fruit will pass through the digestive tract undamaged. When the animal defecates, hours later and usually at some other location, the seed is expelled along with a supply of fertilizer. But if another type of animal eats the fruit, the seed might be destroyed by chewing or the digestive juices.

Birds get sustenance from fruits, help spread the seeds along with their feces which then act as a food supply.

Sweet fruits are thought to be especially appealing to mammals, which tend to have a better sense of taste than birds. Wild chilies on the other hand, are often dispersed by birds. Capsaicin, the compound that makes chilies hot, is thought to be an adaptation for enticing birds. Birds are also fond of some overly mature, fermenting fruits. They sometimes indulge these to the point of becoming too intoxicated to fly.

Sometimes animals such as squirrels and some birds will actually plant the seeds themselves. When they cache (store) seeds in the ground to use in the winter, they often collect more than they need. When they are forgotten or not used, the seeds may germinate into plants.

The second type of seed dispersal involves adaptations for clinging to an animal. Some fruits have evolved hooks or barbs that cling to the animal. Other fruits contain a sticky substance that allows the seed to adhere to an animal as it eats the fruit.

Cockleburs fruits have tiny hooks at its ends that allow it to be carried by animal fur. Velcro was created based on the structure of these exact seeds.

Mechanical dispersal (Ballistic dispersal)

Some plants, such as pea pods, lupines, California poppies, and pansies, have a way of flinging their ripe seeds in all directions with considerable force. They rely on mechanical forces that will eventually cause the seed to be catapulted out when the pods have dried out or touched by another animal.

When the pod starts drying out, tensions are set up in the wall of the pod like a spring and eventually the heat from direct sunlight will cause it to snap and fling speeds all over the place.

The pumpkin-shaped seed capsule of the sandbox tree explodes with a loud sound like a hand grenade when mature spreading the seeds everywhere.

A fully exploded seed capsule of the sandbox tree. The shrapnel consists of flattened, circular seeds that can be flung up to 100 meters (300 ft) away.

The chances of any animal becoming a fossil are extraordinarily remote. The only way that we have a lot of fossils is that there’s been an incredible amount of time and an unbelievable number of animals.

While there are several types of fossil’s, here’s a run down of all the conditions that generally need to be met for fossilization to occur:

Worker termite in Dominican amber.

1. The animal originally had a hard part. Soft-bodied animals have a very low likelihood of preservation.

2. The hard part was still preserved even though the soft parts were dissolved away by bacterial action.

3. Then it got buried in a place where bacterial action doesn’t exist, usually in mud or something similar that is oxygen-free.

4. And then it has to be covered up as soon as possible.

5. However it can’t be just covered up with just anything, it has to be with some kind of a sediment of sandstone, silt or volcanic ash.

As you can tell, the chances of all this happening seem pretty rare, and it is! Fossils can also be frozen or stuck in resin such as amber. Resin fossils often leave absolutely perfect specimens of arthropods (insects and spiders) and more rarely animals such as small lizards. It is thought that the reason trees excrete this resin is to seal its wounds and prevent insects/pathogens from entering.

The only specimen of Leptofoenus pittfieldae thanks to the preservation power of amber.

The picture to the right shows an extinct species of a wasp that dates back from 16 to 20 million years ago. We know of this species only from this single male specimen found on the island of Hispaniola preserved in what is known as Dominican amber.

Dominican amber is transparent and most of the resin fossils in the world have been found there allowing paleontologists to reconstruct ecosystems of ancient tropical rainforests that existed millions of years ago.

One last tidbit I want to share is the fact that when we think of fossils we often think of large animals such as dinosaurs or mammoth’s stuck in glaciers, but most of the fossils we have collected are actually microfossils.

Microfossil of calcareous nannoplankton: The microscopic ancestors of modern plankton.

Microfossils are fossils that are 1mm or smaller and they could be anything from bacteria to fungi to animals and may require an electron microscope to view. These are more important for understanding evolution than visible fossils!

There are obviously no hard parts of bacteria, for example, but through a process known as permineralization, even these microscopic organisms leave an imprint.

When the microscopic organism dies, the empty spaces that are normally filled with liquid or gas during its life become filled with mineral-rich groundwater and the minerals penetrate and fill up those empty spaces. This process occurs in the tiniest of spaces such as within the cell wall of a plant cell and can actually produce very detailed fossils! Who knew?!

Did you know that plants used to live only in water millions of years ago? In fact, they didn’t even look like plants… plants ancestors were simply algae! Eventually, they evolved to find their way onto land and make use of a completely new environment.

Advantages for plants to live on land:

Living on land offered new opportunities such as…

Unlimited sunlight

Abundant CO2

Initially, there were very few pathogens and herbivores.

Challenges for plants to live on land:

Even though living on land had its perks… there were also several problems with it…

Water beads on the waxy cuticle of kale leaves

Plants had to figure out how to regulate water loss within the cells. The waxy covering on top of leaves, called a plant cuticle, evolved on the surface of leaves to reduce water loss. This is why leaves are shiny, waxy and water rolls off. Human epidermis (skin) is similar in its function because our skin serves as a defense against physical damage and infectious organisms and the oils on our skin help us retain water (and keep the epidermis flexible).

Plants had to figure out how to obtain resources from soil and air. There were no vessels that could carry water throughout the plant… so eventually they developed vascular tissue to transport water/sugar within the plant.

Plants had to figure out how to support their body up in the air because algae had no such structures. This gave way to the specialization of the root system and the shoot system.

Plants had to figure out how to reproduce and disperse their offspring without water. This gave way to spores, seeds and fruits for dispersal and pollen for fertilization.

In all plants, the zygote develops into an embryo while attached to and nourished by the parent plant. Plants areembryophytes, with multicellular, dependent embryos.

From algae to moss to seedless, vascular plants to seed-bearing plants to finally… flowering-plants.

The first group of plants that made it on land are best known as BRYOPHYTES.

Bryophytes: the first land plants. They need relatively moist/wet environments to survive.

Early land plants reproduced with spores that would swim (with little whipping tails called flagella) through moist soil and find the female organs. This is why they needed to be in constantly damp environments and early plants only existed near shores and streams.

Without a robust vascular system, plants couldn’t get taller than a couple feet max and most were very close to the ground. Plants eventually evolved to have vessels and those were able to grow very tall and grab more sunlight for photosynthesis (literally overshadowing the competitors) and their spores were able to be blown greater distances due to the greater starting heights.

With vascular tissue, the plants could finally circulate resources throughout the plant. This allowed them to evolve to much larger sizes but they were still seedless and relied on spores.

Seedless, vascular plants such as ferns formed vast ancient forests in low-lying wetlands during the Carboniferous period (360–299 million years ago).

These ancient forests of ferns gave us modern day coal. In the middle of this period, a massive extinction event occurred due to weather changes. When these ferns died, the plants formed peat deposits that eventually formed coal.

Coal, oil, and natural gas are fossil fuels. Oil and natural gas formed from marine organisms; coal formed from seedless plants. Random fact: Insects used to grow much larger because the amount of oxygen in the environment was 40% (double of today).

Depiction of what an ancient forest looked like.

Then gymnosperms came about which were the first to actually have vascular tissue AND SEEDS.

Seeds increased the success rate of the plants because they contained endosperm (nutrients packaged in with the embryo) that gave the seeds the initial resources they needed to reach a decent size to survive.

Seeds are also important because they could be completely isolated from the external atmosphere and were protected from desiccation (dryness), so they could be dormant for long periods of time during droughts and effectively be less vulnerable to extinction. They can literally ride-out the toughest periods and leap back to life when conditions are good. The first seed bearing plants, like the ginkgo and conifers (such as pines and firs), did not produce flowers.

Jacaranda mimosifolia: A beautiful example of a flowering plant.

Angiosperms are flowering plants and are the most successful and diverse of the land plants. Flowers are highly-modified leaves whose main point is for reproduction. Another blog post will be on the way about how they coevolved together with insects.

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